"You can't have any [years] with none, and you can't have any with four, because of our funny calendar," mathemetician Underwood Dudley said.

In many ways the Gregorian calendar, which Pope Gregory XIII ordered the Catholic Church to adopt in 1582, works just like its predecessor, the Julian calendar—with a leap year every four years.

But the Gregorian calendar skips leap year on century years except those divisible by 400. For example, there was no leap year in 1900, but there was one in 2000. This trick keeps the calendar in tune with the seasons.

The result is an ordering of day-dates that repeats itself every 400 years, noted Dudley, a professor emeritus at DePauw University in Indiana, and author of Numerology: Or, What Pythagoras Wrought.

As time marches through the order, some years appear with three Friday the 13ths. Other years have two or, like 2011, one.

"It's just that curious way our calendar is constructed, with 28 days in February and all those 30s and 31s," Dudley said.

When the 400-year order is laid out, another revelation occurs: The 13th falls on Friday more often than on any other day of the week. "It's just a funny coincidence," Dudley said.

Friday the 13th superstitions are rooted in ancient bad-luck associations with the number 13 and the day Friday, said Donald Dossey, a folklore historian and author of Holiday Folklore, Phobias and Fun.

The two unlucky entities ultimately combined to make one super unlucky day.

Dossey traces the fear of 13 to a Norse myth about 12 gods having a dinner party. In walks the uninvited 13th guest, the mischievous Loki. Once there, Loki arranges for Hoder, the blind god of darkness, to shoot Balder the Beautiful, the god of joy and gladness, with a mistletoe-tipped arrow.

"Balder died, and the whole Earth got dark. The whole Earth mourned. It was a bad, unlucky day," Dossey said.

Meanwhile, in ancient Rome, witches reportedly gathered in groups of 12. The 13th was believed to be the devil.

Thomas Fernsler, an associate policy scientist in the Mathematics and Science Education Resource Center at the University of Delaware in Newark, said the number 13 suffers because of its position after 12.

According to Fernsler, numerologists consider 12 a "complete" number. There are 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus, 12 labors of Hercules, 12 tribes of Israel, and 12 apostles of Jesus.

Fernsler said 13's association with bad luck "has to do with just being a little beyond completeness. The number becomes restless or squirmy."

As for Friday, it's well known among Christians as the day Jesus was crucified. Plus, some biblical scholars believe Eve tempted Adam with the forbidden fruit on Friday. Perhaps most significant is a belief that Abel was slain by his brother Cain on Friday the 13th.

Paralyzed with Fear on Friday the 13th

Some people are so paralyzed with superstition on Friday the 13th that they refuse to fly, buy a house, or act on a hot stock tip.

"It's been estimated that [U.S] $800 or $900 million is lost in business on this day because people will not fly or do business they would normally do," said Dossey, who is also the founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina.

Among other services, Dossey's organization counsels clients on how to overcome fear of Friday the 13th, a phobia that he estimates afflicts 17 to 21 million people in the United States.

Symptoms range from mild anxiety to full-blown panic attacks. The latter may cause people to reshuffle schedules or miss an entire day's work.

When it comes to bad luck of any kind, Richard Wiseman—a psychologist at the University of Hertfordshire in Hatfield, England—found that people who consider themselves unfortunate are more likely to believe in superstitions associated with bad luck.

"Their beliefs and behavior are likely to be part of a much bigger worldview," he said. "They will believe that luck is a magical force and that it can ruin their lives."

Where's the 13th Floor?

This fear of 13 can be seen even in how societies are built. More than 80 percent of high-rise buildings lack a 13th floor, for example. And many airports skip the 13th gate. Hospitals and hotels regularly have no room number 13.

On streets in Florence, Italy, the house between number 12 and 14 is addressed as 12 1/2. In France socialites known as the quatorziens (fourteeners) once made themselves available as 14th guests to keep a dinner party from an unlucky fate.